The morphology of the surfaces of the cells of normal and TK-locus mutant mouse embryos, and of sperm from normal and TK-locus mutants, is under investigation in the scanning electron microscope (SEM). Correlated transmission electron microscopy is also employed to amplify and aid in the correct interpretation of scanning electron images. Mutant genes at the TK-locus have the unique property of impairing both embryonic development and the production and function of sperm. Moreover, serological techniques have recently demonstrated cell surface antigens, determined by genes at the TK-locus, on sperm and early embryos, thus establishing the long-held contention that the TK-locus genes specify cell surface properties of sperm and embryos and substantiating the interpretation that mutations at this locus interfere with normal processes of embryonic and cellular differentiation by altering cell surface properties. Therefore, we intend to employ the method of immunoelectron microscopy (SEM) to determine the topography of TK-locus antigens on sperm and embryos and to test whether TK-locus antigens are "differentiation antigens" expressed sequentially on specific groups of cells during embryonic development and play a role in sperm differentiation.